1. Field of the Invention
This invention relates generally to low dielectric constant porous materials and, more particularly, to a process for using porous layers in printed circuit boards and over electronic circuits and supporting substrates.
2. Description of the Related Art
Electronic circuits have inherent limitations in high frequency applications. For example, capacitive loading is increased at higher frequencies because capacitive reactance is directly proportional to frequency. In high frequency circuits capacitive loading becomes so high that a semiconductor chip cannot drive the low reactive impedance. Because these high frequency circuits have historically been power inefficient, their usefulness in airborne and satellite applications is limited.
Foam cored cable technology has been used for years in attempts to reduce capacitive loading of electronic signals. Porous polytetrafluoroethylene (PTFE) materials such as GOR-TEX.TM. (FOR-TEX is a trademark of W. L. Gore & Associates, Inc., of Newark, Del.) have excellent low dielectric constant properties, but unfortunately they absorb process chemicals in their pores during MCM (multi-chip module) or PC (printed circuit) board processing that cannot be removed. Additionally, the fact that the porous material deforms during pressurized processing destroys its porous properties, and therefore the deformed material does not provide the desired consistent low dielectric constant.
In microwave circuit chip structures, certain electrical conductor paths in sensitive areas of the chip circuitry require a dielectric constant close to that of air (1) to work properly in these areas. Chip manufacturers fabricate air bridges to provide the low dielectric constant. These sensitive air bridge structures create processing challenges for both the manufacturer and the end user. Even Teflon polytetrafluoroethylene (Teflon is a trademark of E. I. DuPont de Nemours and Co.), which typically has a dielectric constant between 1.9 and 2.0, can create too much of an impedance loading effect at microwave frequencies.
The DuPont Teflon polytetrafluoroethylene line of materials has intrinsically low dielectric constants and has the necessary high temperature stability for most industrial and military uses. As described in Gore, U.S. Pat. No. 3,962,153, and Gore, U.S. Pat. No. 4,096,227, GOR-TEX material is a Teflon PTFE-based material that is filled with micro-pores that create a foam-like cloth material. Porous PTFE material is generally used in clothes to simultaneously provide warmth and allow perspiration and moisture vapors to vent through its pores. Porous PTFE material is alternatively sold as a filled porous PTFE material. Filling the pores with a material such as an acrylate glue enables use of the material as a laminant film; however, this causes the resultant dielectric constant to degrade and limits the material's usefulness at high frequencies. Unfilled porous PTFE material is an excellent candidate for high frequency circuits because the dielectric constant can be significantly less than that found in other materials. However, in laminant applications and processing, the pores can collapse under pressure and/or be filled with processing materials during fabrication which cannot be easily removed.
Ceramic foam materials are discussed, for example, in "Giants in advanced ceramics," Ceramic Industry, vol. 141, 47 (August 1993), and Te-Kao Wu, "Dielectric properties measurement of substrate and support materials," Microwave and Optical Technology Letters, vol. 3, 283-286 (August 1990). These materials, however, are inflexible and not fabricated in large area sections. Because the ceramic foam materials are inflexible, they can be used as substrates but cannot be used in overlay technologies that are not perfectly flat. Because ceramic foam materials are fragile, they are typically not capable of withstanding PC board lamination processing in their intrinsic state. Furthermore, ceramic foam materials can be filled with processing materials which degrade their low dielectric constant.
Another problem common to both porous PTFE material and ceramic foam material is that if the porous/foam material is to be used in any process in which a metallization layer is applied over the material, the metal tends to extend into the pores and it is thus difficult to achieve a smooth and continuous metal surface for use in high frequency applications where skin-effect related losses are of importance.
Few intrinsically low dielectric constant materials can withstand the high temperatures in commercial and military processing. Most of the low dielectric constant materials that can withstand higher temperatures are only absorptive below the 200 nanometer wavelength region of the light spectrum, and thus it is nearly impossible to process those materials using ion argon and other CW (continuous wave) lasers.
Polymer materials in high frequency circuits are preferably laser ablatable by ultraviolet light in order to form the via openings through which different layers of metallization are connected. Laser processing (ablation, photoresist exposure, etc.) is normally done with several passes of the laser beam with a power ranging from 0.5 to 2.0 watts, with a preferred maximum power level being about 1.5 watts. Thus, when a dielectric layer is characterized as being laser ablatable, it means that such layer can be totally removed by progressive passes of a laser beam of this power level, and when it is characterized as not being laser ablatable, it means that a layer is not completely removed by progressive passes of such a laser beam. One method of adaptive laser ablation is discussed in Eichelberger et al., U.S. Pat. No. 4,835,704. Most polytetrafluoroethylene materials are not laser ablatable and generally do not easily adhere to other materials. A method of modifying the ultraviolet absorption characteristics of a polymer material with the addition of an ultraviolet absorbing dye is discussed is Cole et al., U.S. Pat. No. 5,169,678.
To minimize the complexity and cost of equipment for fabricating high density interconnect structures, it is considered desirable to be able to do all laser processing at a single frequency in order that only a single laser is required. Accordingly, preferred materials are those which can be processed at a laser frequency of 351 nm. This frequency was selected in accordance with the characteristics of desirable dielectric layers such as Kapton.TM. polyimide (Kapton is a trademark of E. I. DuPont de Nemours and Co.) and the fact that there are commercial photoresists which can be processed at this frequency. Ultem.TM. polyetherimide resin (Ultem is a registered trademark of General Electric Co.) has been used as an adhesive layer in this high density interconnect structure for bonding Kapton polyimide to the underlying structures. The Ultem polyetherimide resin is laser ablatable at 351 nm.
PC boards typically have dielectric constants ranging from 2 through 5, and nonporous ceramics can have dielectric constants as high as 9. It would be desirable to have a low dielectric constant PC board material for use in high frequency and high speed electronics.